Wouldn't you know it, the next day it started right up a ran all day without any problem. Now I don't know what to tell the service dept.

It's always like that! Somethings broke and when you take it to the dealer it's working fine.

I believe the Onan generators have a 3 year warranty. Make sure you have done the 25 hr oil change prior and have registered the generator on the Onan website..
Since there is the 36 trouble code, it will have to be addressed. My authorized Onan service center was across the street from the dealer. The dealer didn't have authorized techs. They did a very intensive diagnostic check to determine my fuel pump was faulty.

I wonder if vapor lock is a contributing factor? You said you were running in hot weather for an extended period of time. Maybe the same will be required to recreate the problem.

It was just about as hot the next day. And it ran for longer still. Also, the first day it stopped running when the RV's gas tank was at 3/4 Full, but the next day it ran all day and I arrived when the gas was at 1/2 Full.

When traveling through Texas if you don't run your Genset and AC all day in the coach you'll arrive to your CG with the interior temp in the high 80's to near 90 degrees. And, it will take until 3 am to get the coach cool enough to sleep comfortably.

So, that's why we ran the genset and AC on both of those days. The first day when the genset quit on me the interior of the coach was 89 degrees when we arrived. And, only an hour had passed between the time the genset quit and we arrived at our CG.

Luckily, we were traveling in New Mexico that day (it was 104 degrees!) but there's very low humidity and the night cooled off so we finally saw mid-70's inside by the time we went to bed.

The next day it was cooler in Texas - only 94 degrees - but very high humidity. The generator cooperated that day and kept the coach at mid-70's all day as we drove the 360 miles to our CG. That makes a big difference in comfort inside on warm days.

I'm wondering if it was some contaminant from the genset fuel system. I haven't been religious about starting it and running it under load for most of the winter months - I did run it, but every 30-days??? I'm not sure about that.

When on a camping trip, outside of these two hot days, we haven't needed to run it all that much. Just a lunch stop here and there.

I'll have the dealer take a look, but I'm confident he'll say nothing could be found.